feat: don't create temporary objects for serialization

Fix: #646

Author: gjasny

core/CMakeLists.txt

@@ -10,8 +10,8 @@ add_library(core
   src/gauge.cc
   src/histogram.cc
   src/info.cc
+  src/iovector.cc
   src/registry.cc
-  src/serializer.cc
   src/summary.cc
   src/text_serializer.cc
 )

core/include/prometheus/collectable.h

@@ -1,12 +1,7 @@
 #pragma once
 
-#include <vector>
-
 #include "prometheus/detail/core_export.h"
-
-namespace prometheus {
-struct MetricFamily;
-}
+#include "prometheus/serializer.h"
 
 namespace prometheus {
 
@@ -19,7 +14,7 @@ class PROMETHEUS_CPP_CORE_EXPORT Collectable {
   virtual ~Collectable() = default;
 
   /// \brief Returns a list of metrics and their samples.
-  virtual std::vector<MetricFamily> Collect() const = 0;
+  virtual void Collect(const Serializer& out) const = 0;
 };
 
 }  // namespace prometheus

core/include/prometheus/family.h

@@ -4,15 +4,14 @@
 #include <mutex>
 #include <string>
 #include <unordered_map>
-#include <vector>
 
 #include "prometheus/client_metric.h"
 #include "prometheus/collectable.h"
 #include "prometheus/detail/core_export.h"
 #include "prometheus/detail/future_std.h"
 #include "prometheus/detail/utils.h"
 #include "prometheus/labels.h"
-#include "prometheus/metric_family.h"
+#include "prometheus/serializer.h"
 
 // IWYU pragma: no_include "prometheus/counter.h"
 // IWYU pragma: no_include "prometheus/gauge.h"
@@ -140,7 +139,7 @@ class PROMETHEUS_CPP_CORE_EXPORT Family : public Collectable {
   /// Collect is called by the Registry when collecting metrics.
   ///
   /// \return Zero or more samples for each dimensional data.
-  std::vector<MetricFamily> Collect() const override;
+  void Collect(const Serializer& out) const override;
 
  private:
   std::unordered_map<Labels, std::unique_ptr<T>, detail::LabelHasher> metrics_;

core/include/prometheus/iovector.h

@@ -0,0 +1,27 @@
+#pragma once
+
+#include <cstddef>
+#include <cstdint>
+#include <string>
+#include <vector>
+
+#include "prometheus/detail/core_export.h"
+
+namespace prometheus {
+
+struct PROMETHEUS_CPP_CORE_EXPORT IOVector {
+  using ByteVector = std::vector<std::uint8_t>;
+
+  bool empty() const;
+
+  std::size_t size() const;
+
+  std::size_t copy(std::size_t offset, void* buffer,
+                   std::size_t bufferSize) const;
+
+  void add(const std::string& str, std::size_t chunkSize);
+
+  std::vector<ByteVector> data;
+};
+
+}  // namespace prometheus

core/include/prometheus/metric_family.h

@@ -13,6 +13,5 @@ struct PROMETHEUS_CPP_CORE_EXPORT MetricFamily {
   std::string name;
   std::string help;
   MetricType type = MetricType::Untyped;
-  std::vector<ClientMetric> metric;
 };
 }  // namespace prometheus

core/include/prometheus/registry.h

@@ -9,7 +9,7 @@
 #include "prometheus/detail/core_export.h"
 #include "prometheus/family.h"
 #include "prometheus/labels.h"
-#include "prometheus/metric_family.h"
+#include "prometheus/serializer.h"
 
 namespace prometheus {
 
@@ -79,7 +79,7 @@ class PROMETHEUS_CPP_CORE_EXPORT Registry : public Collectable {
   /// function.
   ///
   /// \return Zero or more metrics and their samples.
-  std::vector<MetricFamily> Collect() const override;
+  void Collect(const Serializer& out) const override;
 
   /// \brief Removes a metrics family from the registry.
   ///

core/include/prometheus/serializer.h

@@ -1,9 +1,6 @@
 #pragma once
 
-#include <iosfwd>
-#include <string>
-#include <vector>
-
+#include "prometheus/client_metric.h"
 #include "prometheus/detail/core_export.h"
 #include "prometheus/metric_family.h"
 
@@ -12,9 +9,10 @@ namespace prometheus {
 class PROMETHEUS_CPP_CORE_EXPORT Serializer {
  public:
   virtual ~Serializer() = default;
-  virtual std::string Serialize(const std::vector<MetricFamily>&) const;
-  virtual void Serialize(std::ostream& out,
-                         const std::vector<MetricFamily>& metrics) const = 0;
+
+  virtual void SerializeHelp(const MetricFamily& family) const = 0;
+  virtual void SerializeMetrics(const MetricFamily& family,
+                                const ClientMetric& metric) const = 0;
 };
 
 }  // namespace prometheus

core/include/prometheus/text_serializer.h

@@ -1,19 +1,29 @@
 #pragma once
 
+#include <cstddef>
 #include <iosfwd>
-#include <vector>
 
+#include "prometheus/client_metric.h"
 #include "prometheus/detail/core_export.h"
+#include "prometheus/iovector.h"
 #include "prometheus/metric_family.h"
 #include "prometheus/serializer.h"
 
 namespace prometheus {
 
 class PROMETHEUS_CPP_CORE_EXPORT TextSerializer : public Serializer {
  public:
-  using Serializer::Serialize;
-  void Serialize(std::ostream& out,
-                 const std::vector<MetricFamily>& metrics) const override;
+  TextSerializer(IOVector& ioVector);
+
+  void SerializeHelp(const MetricFamily& family) const override;
+  void SerializeMetrics(const MetricFamily& family,
+                        const ClientMetric& metric) const override;
+
+ private:
+  void Add(const std::ostringstream& stream) const;
+
+  IOVector& ioVector_;
+  static constexpr std::size_t chunkSize_ = 1 * 1024 * 1024;
 };
 
 }  // namespace prometheus

core/src/family.cc

@@ -11,6 +11,7 @@
 #include "prometheus/gauge.h"
 #include "prometheus/histogram.h"
 #include "prometheus/info.h"
+#include "prometheus/metric_family.h"
 #include "prometheus/summary.h"
 
 namespace prometheus {
@@ -86,22 +87,24 @@ const Labels& Family<T>::GetConstantLabels() const {
 }
 
 template <typename T>
-std::vector<MetricFamily> Family<T>::Collect() const {
+void Family<T>::Collect(const Serializer& out) const {
   std::lock_guard<std::mutex> lock{mutex_};
 
   if (metrics_.empty()) {
-    return {};
+    return;
   }
 
   auto family = MetricFamily{};
   family.name = name_;
   family.help = help_;
   family.type = T::metric_type;
-  family.metric.reserve(metrics_.size());
+
+  out.SerializeHelp(family);
+
   for (const auto& m : metrics_) {
-    family.metric.push_back(std::move(CollectMetric(m.first, m.second.get())));
+    auto&& metric = CollectMetric(m.first, m.second.get());
+    out.SerializeMetrics(family, metric);
   }
-  return {family};
 }
 
 template <typename T>

core/src/iovector.cc

@@ -0,0 +1,58 @@
+#include "prometheus/iovector.h"
+
+#include <algorithm>
+#include <iterator>
+#include <memory>
+#include <numeric>
+
+namespace prometheus {
+
+bool IOVector::empty() const { return data.empty() || !size(); }
+
+std::size_t IOVector::size() const {
+  return std::accumulate(begin(data), end(data), std::size_t{0},
+                         [](std::size_t size, const ByteVector& chunk) {
+                           return size + chunk.size();
+                         });
+}
+
+std::size_t IOVector::copy(std::size_t offset, void* buffer,
+                           std::size_t bufferSize) const {
+  std::size_t copied = 0;
+  for (const auto& chunk : data) {
+    if (offset >= chunk.size()) {
+      offset -= chunk.size();
+      continue;
+    }
+
+    auto chunkSize = std::min(chunk.size() - offset, bufferSize - copied);
+    std::copy_n(chunk.data() + offset, chunkSize,
+                reinterpret_cast<std::uint8_t*>(buffer) + copied);
+    copied += chunkSize;
+    offset = 0;
+
+    if (copied == bufferSize) {
+      break;
+    }
+  }
+  return copied;
+}
+
+void IOVector::add(const std::string& str, std::size_t chunkSize) {
+  std::size_t size = str.size();
+  std::size_t offset = 0;
+
+  while (size > 0U) {
+    if (data.empty() || data.back().size() >= chunkSize) {
+      data.emplace_back();
+      data.back().reserve(chunkSize);
+    }
+    auto&& chunk = data.back();
+    const auto toAdd = std::min(size, chunkSize - chunk.size());
+    chunk.insert(chunk.end(), str.data() + offset, str.data() + offset + toAdd);
+
+    size -= toAdd;
+    offset += toAdd;
+  }
+}
+}  // namespace prometheus